https://doi.org/10.5194/nhess-2020-385 Preprint. Discussion started: 21 December 2020 c Author(s) 2020. CC BY 4.0 License. 1 2 3 4 5 6 7 8 9 10 11 12 Vulnerability and Site Effects in Earthquake Disasters in Armenia 13 (Colombia). II – Observed Damages and Vulnerability 14 15 16 by 17 18 19 Francisco J. Chávez-García1, Hugo Monsalve-Jaramillo2, Joaquín Vila-Ortega3 20 21 22 23 24 25 26 27 28 29 30 1Professor Instituto de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán 04510 31 CDMX, México [email protected] 32 33 34 2 Correspondence to: Professor Facultad de Ingeniería, Universidad del Quindío, Cra. 15 #12N, Armenia, Quindío, 35 Colombia [email protected] 36 37 38 3Professor Facultad de Ingeniería, Universidad del Quindío, Cra. 15 #12N, Armenia, Quindío, Colombia 39 [email protected] 40 41 42 43 44 45 46 47 48 49 50 51 52 53 1 https://doi.org/10.5194/nhess-2020-385 Preprint. Discussion started: 21 December 2020 c Author(s) 2020. CC BY 4.0 License. 54 Abstract 55 56 Damage in Armenia, Colombia, for the 1999 (Mw6.2) event was disproportionate. We analyse the damage report as a 57 function of number of storeys and construction age. We recovered two vulnerability evaluations made in Armenia in 1993 58 and in 2004. We compare the results of the 1993 evaluation with damages observed in 1999 and show that the vulnerability 59 evaluation made in 1993 could have predicted the relative frequency of damage observed in 1999. Our results show that 60 vulnerability of the building stock was the major factor behind damage observed in 1999. Moreover, it showed no 61 significant reduction between 1999 and 2004. 62 63 Key words: earthquake damage; vulnerability; construction type; construction age; building inventory. 64 65 1 Introduction 66 67 Destructive earthquakes occur relatively frequently in Colombia (the first reported event dates from 1551, Espinosa, 68 2003). However, the development of earthquake engineering began only relatively recently, punctuated by several major, 69 significant events. The first building code in the country was published in 1984 (CCCSR-84, 1984), partly as a result of 70 the heavy toll caused by the Popayán earthquake in March, 1983 (Ingeominas, 1986). Increasing building requirements 71 have improved earthquake resistance, for example phasing out non engineered construction. The development of 72 earthquake engineering has led to a decrease in the vulnerability of buildings in Colombia but progress has been slow, in 73 pace with the development of building codes. In addition, as favoured construction styles evolve, additional challenges 74 appear. For example, the cost of land pushes current housing projects consisting of tall concrete structures for which there 75 is little experience regarding their seismic behaviour in that country. Instrumenting some of those buildings to analyze 76 their motion during small earthquakes would provide useful data and may eventually become a necessity (e.g., Meli et 77 al., 1998). Meanwhile, it is important to learn as much as possible from past destructive events. 78 79 Damage evaluation after large earthquakes is recognized as a primary input to understand structural response subject to 80 dynamic excitations. It offers valuable data on the behaviour of structures to actual seismic motion. In addition to very 81 significant efforts like GEER (Geotechnical Extreme Events Reconnaissance, 2020), local initiatives have contributed 82 significantly to understand damage occurrence, especially in relation to site effects (e.g., Montalva et al., 2016; Fernández 83 et al., 2019). 84 85 One seismic event that has had a long lasting impact in Colombia is the January 25, 1999, earthquake in the Quindío 86 department, close (18 km) to the city of Armenia. This relatively small (Mw6.2), normal fault earthquake had profound 87 economic and social consequences in the country. There was only one accelerograph in Armenia, and it recorded PGA of 88 518/580/448 gal in the EW/NS/Z components. Strong ground motion duration was very short (smaller than 5 s) and 89 ground motion energy peaked at periods shorter than 0.5 s. The source of the main shock and aftershocks was studied in 90 Monsalve-Jaramillo and Vargas-Jiménez (2002), while macroseismic observations were presented in Cardona (1999). 91 The city of Armenia sustained heavy damage (maximum intensity was IX in EMS-96 scale): 2000 casualties and 10,000 92 injuries due to the collapse of 15,000 houses, with a further 20,000 houses severely damaged (SIQ, 2002). Site effect 93 evaluation during this event in Armenia was addressed by Chávez-García et al. (2018). Earthquake and ambient noise 94 data were analysed with the objective of characterizing local amplification due to soft surficial layers using a variety of 95 techniques. The results showed that, while local amplification contributed significantly to destructive ground motion, 96 observed damage distribution in 1999 was incompatible with the rather small variations in dominant frequency and 97 maximum amplification throughout the city. 98 99 Chávez-García et al. (2018) referred to the damage distribution observed for the 1999 earthquake but no data were 100 analysed in that paper. In this paper, we present an analysis of damage observed during the 1999 earthquake. Earthquake 101 damage data is analysed in relation to geology and to the site classes defined in the microzonation map of Armenia 102 (Asociación Colombiana de Ingeniería Sísmica, 1999). In addition, the city of Armenia offers a very uncommon 103 advantage in Latin America. Two vulnerability studies have been conducted in the city, one in 1993 and one in 2004. We 104 compare the 1999 damage distribution to vulnerability estimated in 1993 for the small downtown district of the city where 105 the two data sets overlap. The comparison of the two vulnerability studies, in 1993 and in 2004, allows an assessment of 106 the changes in vulnerability in the city as a consequence of a destructive earthquake, even if the method used was different 107 and the studied zones overlap only partially. We show that building vulnerability was the main factor behind the heavy 108 damage toll in Armenia during the 1999 earthquake. Our results substantiate the improvement of engineering practice 109 with time and provide evidence of the efficacy of simple methods to evaluate vulnerability. However, they also strike an 110 alarm bell as they show that vulnerability in Armenia remains high. Our results offer an unusually complete analysis of 111 the major factors behind seismic risk in a typical medium size city in Colombia. Seismic risk mitigation in Armenia, and 112 in similar midsize cities in Latin America, requires an increase in the number of permanent seismic stations and support 113 of additional efforts to improve our understanding of moderate size seismic events. 114 115 2 https://doi.org/10.5194/nhess-2020-385 Preprint. Discussion started: 21 December 2020 c Author(s) 2020. CC BY 4.0 License. 116 2 Colombian Building Codes and Practice Evolution 117 118 This paper will obviate a discussion of the geological setting of Armenia, as it can be found in Chávez-García et al. (2018). 119 The coffee growing region was occupied during the second half of the 19th century. For this reason, data on historical 120 earthquakes is scarce, even though it is located in a zone of high seismic hazard (the current Colombian building code 121 prescribes a PGA of 0.25 g for Armenia for a return period of 475 yr). During the 20th century, about eight earthquakes 122 occurred in the region producing intensities as large as IX (Espinosa, 2011). 123 124 Before 1960, construction in this region consisted mainly of bahareque and unreinforced masonry. In Colombia, 125 bahareque refers to structures that use guadua (a local variety of bamboo) for the skeleton elements. Walls are made using 126 a guadua-based mat, covered with mud mixed with dung as bonding agent. At about 1960, reinforced concrete frames 127 began to be used but Colombia lacked a building code until 1984, although conscientious engineers followed guidelines 128 from international codes, mostly American ones. Between 1977 and 1984 design practice for those structures shifted from 129 the elastic method to ultimate strength design. Unfortunately, this allowed construction companies to decrease the quantity 130 of steel reinforcement. Until 1984, no seismic provisions were considered. 131 132 A major milestone was the Popayán earthquake of March 31, 1983 (ML5.5). This small, shallow event caused major 133 destruction in Popayán, where important Spanish heritage sites were severely damaged. Although restricted in extension, 134 the heavy damage gave the final push for the adoption of a national building code including seismic provisions in 1984. 135 This code had been promoted since the end of 1970’s by Asociación Colombiana de Ingeniería Sísmica (Colombian 136 Association for Earthquake Engineering), founded on 1975. A major consequence of the 1984 code was to eliminate new 137 construction using unreinforced masonry. This code was replaced by a new version in 1998. The effects of two events in 138 1995 (Feb 8, Mw6.6, Aug 19, Mw6.5) convinced engineers that lateral drift requirements in the 1984 code were too 139 lenient and stricter requirements were incorporated. 140 141 Only a few months passed between publication of the 1998 building code and the occurrence of the 1999, Armenia, 142 earthquake. Some of the shortcomings identified during this event were addressed in improvements to the code published 143 in 2010; requisites for irregular buildings with weak storeys, short columns, p-Δ effects, and torsion related problems 144 among others. Microzonation of cities with more than 100,000 people became mandatory.